Heat-softenable toners are widely used in imaging methods such as electrostatography, wherein electrically charged toner is deposited imagewise on a dielectric or photoconductive element bearing an electrostatic latent image. Most often in such methods, the toner is then transferred to a surface of another substrate, such as, e.g., a receiver sheet comprising paper or a transparent film, where it is then fixed in place to yield the final desired toner image.
When heat-softenable toners, comprising, e.g., thermoplastic polymeric binders, are employed, the usual method of fixing the toner in place involves applying heat to the toner once it is on the receiver sheet surface to soften it and then allowing or causing the toner to cool.
One such well-known fusing method comprises passing the toner-bearing receiver sheet through a nip formed by a pair of opposing rolls, at least one of which (usually referred to as a fuser roll) is heated and contacts the toner-bearing surface of the receiver sheet in order to heat and soften the toner. The other roll (usually referred to as a pressure roll) serves to press the receiver sheet into contact with the fuser roll. In some other fusing methods, the configuration is varied and the “fuser roll” or “pressure roll” takes the form of a flat plate or belt. The description herein, while generally directed to a generally cylindrical fuser roll in combination with a generally cylindrical pressure roll, is not limited to fusing systems having members with those configurations. For that reason, the term “fuser member” is generally used herein in place of “fuser roll” and the term “pressure member” in place of “pressure roll”.
The fuser member usually comprises a rigid core covered with a resilient material, which will be referred to herein as a “base cushion layer.” The resilient base cushion layer and the amount of pressure exerted by the pressure member serve to establish the area of contact of the fuser member with the toner-bearing surface of the receiver sheet as it passes through the nip of the fuser member and pressure members. The size of this area of contact helps to establish the length of time that any given portion of the toner image will be in contact with and heated by the fuser member. The degree of hardness (often referred to as “storage modulus”) and stability thereof, of the base cushion layer are important factors in establishing and maintaining the desired area of contact.
In some previous fusing systems, it has been advantageous to vary the pressure exerted by the pressure member against the receiver sheet and fuser member. This variation in pressure can be provided, for example in a fusing system having a pressure roll and a fuser roll, by slightly modifying the shape of the pressure roll. The variance of pressure, in the form of a gradient of pressure that changes along the direction through the nip that is parallel to the axes of the rolls, can be established, for example, by continuously varying the overall diameter of the pressure roll along the direction of its axis such that the diameter is smallest at the midpoint of the axis and largest at the ends of the axis, in order to give the pressure roll a sort of “bow tie” or “hourglass” shape. This will cause the pair of rolls to exert more pressure on the receiver sheet in the nip in the areas near the ends of the rolls than in the area about the midpoint of the rolls. This gradient of pressure helps to prevent wrinkles and cockle in the receiver sheet as it passes through the nip. Over time, however, the fuser roll begins to permanently deform to conform to the shape of the pressure roll and the gradient of pressure is reduced or lost, along with its attendant benefits. It has been found that permanent deformation (alternatively referred to as “creep”) of the base cushion layer of the fuser member is the greatest contributor to this problem.
Particulate inorganic fillers have been added to base cushion layers to improve mechanical strength and thermal conductivity. High thermal conductivity is advantageous when the fuser member is heated by an internal heater, so that the heat can be efficiently and quickly transmitted toward the outer surface of the fuser member and toward the toner on the receiver sheet it is intended to contact and fuse. High thermal conductivity is not so important when the roll is intended to be heated by an external heat source.
Polyfluorocarbon elastomers, such as vinylidene fluoride-hexafluoropropylene copolymers, are tough, wear resistant and flexible elastomers that have excellent high temperature resistance, but relatively high surface energies, which compromises toner release.
Fluorocarbon resins like polytetrafluoroethylene (PTFE) or fluorinated ethylenepropylene (FEP) are fluorocarbon plastics which have excellent release characteristics due to very low surface energy. Fluorocarbon resins are, however, less flexible and elastic than fluorocarbon elastomers and are therefore not suitable alone as the surface of the fuser roller.
U.S. Pat. No. 4,568,275 discloses a fuser roll having a layer of fluorocarbon elastomer and a fluorinated resin powder. However, the fluorocarbon elastomer that is disclosed is water dispersible and it is known that the mixture phase separates on coating so that the fluorinated resin that is used comes to the surface of the layer.
U.S. Pat. No. 5,253,027 discloses a fluorinated resin in a silicone elastomer. However, composites of this type exhibit unacceptable swell in the presence of silicone release oil.
U.S. Pat. No. 5,599,631 discloses a fuser roll having a layer of a fluorocarbon elastomer and a fluorocarbon resin. The drawback of this type of material is that the fluorocarbon resin powder tends to phase separate from the fluorocarbon elastomer thereby diminishing toner release.
U.S. Pat. No. 4,853,737 discloses a fuser roll having an outer layer comprising cured fluorocarbon elastomers containing pendant amine functional polydimethylsiloxane that are covalently bonded to the backbone of the fluorocarbon elastomer. However, the amine functional polydimethylsiloxane tends to phase separate from the fluorocarbon elastomer.
U.S. Pat. No. 5,582,917 discloses a fuser roll having a surface layer comprising a fluorocarbon-silicone polymeric composition obtained by heating a fluorocarbon elastomer with a fluorocarbon elastomer curing agent in the presence of a curable polyfunctional poly(C1-6 alkyl) siloxane polymer. However, the resulting interpenetrating network (IPN) has relatively high coefficient of friction and relatively low mechanical strength. After a period of use, the release property of the roller degrades and paper jams begin to occur.
U.S. Pat. No. 5,547,759 discloses a fuser roll having a release coating layer comprising an outermost layer of fluorocarbon resin uniquely bonded to a fluoroelastomer layer by means of a fluoropolymer containing a polyamide-imide primer layer. Although the release coating layer has relatively low surface energy and good mechanical strength, the release coating layer lacks flexibility and elastic properties and can not produce high quality of images.
In addition, sintering the fluorocarbon resin layer is usually accomplished by heating the coated fuser member to temperatures of approximately 350° C. to 400° C. Such high temperatures can have a detrimental effect on the underlying base cushion layer which normally comprises a silicone rubber layer. It would be desirable to provide a fuser member with an overcoat layer comprising a fluorocarbon resin layer without depolymerizing the silicone base cushion layer on heating.
Polysiloxane elastomers have relatively high surface energy and relatively low mechanical strength, but are adequately flexible and elastic and can produce high quality fused images. After a period of use, however, the self release property of the roller degrades and offset begins to occur. Application of a polysiloxane fluid during roller use enhances the ability of the roller to release toner, but shortens roller life due to oil absorption. Oiled portions tend to swell and wear and degrade faster.
One type of material that has been widely employed in the past to form a resilient base cushion layer for fuser rolls is a condensation-crosslinked siloxane elastomer. Disclosure of filled condensation-cured poly(dimethylsiloxane)(“PDMS”) elastomers for fuser rolls can be found, for example, in U.S. Pat. Nos. 4,373,239; 4,430,406; and 4,518,655. U.S. Pat. No. 4,970,098 to Ayala-Esquillin et al. teaches a condensation cross-linked diphenylsiloxane-dimethylsiloxane elastomer having 40 to 55 weight percent zinc oxide, 5 to 10 weight percent graphite, and 1 to 5 weight percent ceric dioxide.
A widely used siloxane elastomer is a condensation-crosslinked PDMS elastomer, which contains about 32–37 volume percent aluminum oxide filler and about 2–6 volume percent iron oxide filler, and is sold under the trade name, EC4952, by the Emerson Cummings Co., U.S.A.
It has been found that fuser rolls containing EC4952 cushion layers exhibit serious stability problems over time of use, i.e., significant degradation, creep, and changes in hardness, that greatly reduce their useful life. Nevertheless, materials such as EC4952 initially provide very suitable resilience, hardness, and thermal conductivity for fuser roll cushion layers.
U.S. Pat. No. 5,595,823 discloses toner fusing members which have a substrate coated with a fluorocarbon random copolymer containing aluminum oxide. Although these toner fusing members have proved effective and have desirable thermal conductivity, they have a problem in that there can be toner contamination. The advantage of using the cured fluorocarbon thermoplastic random copolymer compositions is that they are effective for use with toner release agents which typically include silicone.
U.S. Pat. No. 5,464,698 discloses toner fusing members which have a substrate coated with a fluorocarbon random copolymer containing tin oxide. Although these toner fusing members have proved effective and have desirable thermal conductivity, they have a problem in that there can be toner contamination.
Commonly-assigned U.S. Pat. No. 6,041,210 describes a toner fusing member having an overcoat layer including electrically conductive fine powders having a weight percent between about 30 to 80 weight percent. Although these toner fusing members have proved effective in suppressing electrostatic charge build up, they have a problem in that there can be toner contamination.
Thus, it has been extremely difficult to provide a fuser roller with, at the same time, good wear resistance, good release property, low coefficient of friction and low oil swell when exposed to release oil. It is toward a solution to this problem that the present invention is directed. It would be desirable to provide a method for making a fuser member with an overcoat layer that includes a fluorocarbon thermoplastic random copolymer that can be cured at lower temperatures while providing good mechanical properties and low toner contamination.